Scientists at University Health Network have recently developed an in-vivo optical dosimeter platform used for the real-time monitoring of radiation dose delivered to clinical radiotherapy patients. Their dosimeter platform is described in U.S. Pat. No. 7,399,977. It enables radiotherapy techniques such as MR-guided Radiotherapy (MRgRT) and MR-guided brachytherapy and imminent quality assurance requirements for the field. The unique advantage of this platform is in its MR-compatible features, water-equivalent composition and real-time readout.
Radiotherapy is commonly used alone or in conjunction with other therapies to cure or control malignant disease. It works by delivering a predetermined dose of ionizing radiation to the desired volume, while limiting the dose deposited in the surrounding healthy organs. Thus the outcome of treatment is highly dependent on the accuracy of the prescribed dose and fractionation, as well as the spatial distribution. While the modern dose calculation and delivery techniques are becoming more complex, there is also a strong trend towards dose escalation, steeper dose gradients close to the tumor, and hypofractionation—aspects of treatment that can lead to serious complications if errors were made during delivery. These errors can occur due to internal and external motion artifacts (i.e. patient movement, breathing, etc), dose calculation and human mistakes.
Various types of dosimeters are already used in radiotherapy to perform quality assurance of the beam characteristics, and some are capable of monitoring dose delivery during treatment. Unfortunately the reasons individual treatment monitoring is not currently performed as part of standard treatment are instrumentation and implementation complexity and expense.
Furthermore, verification of delivered dose is appealing not only from a quality assurance point of view, but also from a point of investigative and clinical research. The list of applications may be, but is not limited to: Quality assurance of dose during first/weekly/every fractions measured on patient's skin, Quality assurance of dose at the site of the tumor, Quality assurance of dose at the site of organ at risk, Investigative dosimetry during novel applications and clinical trials including dose escalation and hypofractionation (SBRT), alternative delivery techniques (VMAT), gated delivery (monitoring organ motion and breathing), Brachytherapy for permanent seed implants (prostate, breast), Brachytherapy for high-dose-rate applications (prostate, esophagus, lung, cervix) both in organs at risk and tumor tissue, Novel MR-guided techniques (MR-guided HDR for prostate, MR-guided LINACs, MR-guided cobalt units, etc.).
There is a clinical need for monitoring of dose delivered to patients during radiotherapy procedures to treat oncological disease. Ideally the radiation dose measured at any point in the patient should be as per the treatment plan in order for the treatment to be successful (defined as control of oncological disease with minimal acute and long-term side-effects). The current standard for radiation treatment typically consists of imaging, planning the treatment based on the 3-D volumetric (sometimes 2-D projection) imaging data, followed by treatment. Variables such as internal and external motion, patient placement, dose calculation inconsistencies and human error have been identified as causes of misdelivery of the ideal planned treatment. Radiotherapy quality assurance (i.e. ensuring delivered dose is as planned at any point) is needed by radiation oncologists and physicists. However, implementing this step on every patient with the current commercially available tools is complex and expensive. This is complicated further by the growing field of MR-guided radiotherapy and MR-guided brachytherapy (MRgRT) which requires MR-compatible QA tools.
Current commercially-available in-vivo dosimeters include TLD's (thermo-luminescent dosimeters), MOSFET's (Metal Oxide Semiconductor Field Effect Transistors) diodes, ion-chambers, and scintillators (see “Competitive and Complementary Technology Landscape”). All of these technologies (except TLD's and scintillators) rely on electronic readout of either accumulated charge, induced current, or a change in bias voltage. Performing such measurements requires metallic conducting components within the dosimeter and signal conduit (e.g. wire), which are generally of high atomic number (Z). As a result these components interact with the ionizing radiation field in a much different way than the tissue it displaced, thus altering the dose distribution within the patient. For diodes this issue can create as much as a 30% error to local dose distribution due to their size, density and composition. Although MOSFET dosimeters are much smaller (i.e. ˜1-2 mm) they still exhibit non-tissue equivalence and thus must be calibrated at the exact beam energy in which they will be used (which is not always known a-priori). This complicates and prolongs work-flow and may yield incorrect results.